Was the ignition the same for the other dyno runs or was it fitted later? I ask this as too much retard shouldn't start to be an issue until running very low or if retarding too early. If you look at Cambridge Lambrettas setup recommendations for their kitted bikes with Varitronic advance/retard, they often suggest that kits with expansions start at 19 dbtdc and retard to considerably less... perhaps less than 15 degrees at higher RPM. This makes me think that your situation could be that it is retarding too far too soon, rather than simply too far (rate vs. starting point), which is why some retard profiles better suit different motor/pipe specs (e.g. Agusto 6000, 7000, 8000 units).

Peak at around 6500...? I'm surprised by it being at such low RPM, even with a touring tune. My old touring tune TS1 was detuned from standard but (base of cylinder machined to reduce transfer timing) still had best power between 7500-8000. When trying pipes with lower RPM range I simply found that they cut a TS1 short, before they reach the revs that most TS1s perform best at.

Were you running this same ignition when it was making better power with an expansion?

Peak at around 6500...? I'm surprised by it being at such low RPM, even with a touring tune. My old touring tune TS1 was detuned from standard but (base of cylinder machined to reduce transfer timing) still had best power between 7500-8000. When trying pipes with lower RPM range I simply found that they cut a TS1 short, before they reach the revs that most TS1s perform best at.

Were you running this same ignition when it was making better power with an expansion?